Subaru Witnesses Galactic Cannibalism in Action

Subaru telescope has witnessed a large galaxy in the act of devouring a
small companion galaxy in a new image obtained by Yoshiaki Taniguchi
(Tohoku University), Shunji Sasaki (Tohoku University), Nicolas Scoville
(California Institute of Technology) and colleagues. The evidence is a
wispy band of stars extending over 500 thousand light years, the
faintest and longest known example of its kind.

Current theories of galaxy formation suggest that large galaxies like
the Milky Way grow by consuming smaller dwarf galaxies. Evidence of this
process can be found in our own galactic neighborhood. Some stars in the
Milky Way appear to have once belonged to a small nearby galaxy called
the Sagitarius Dwarf. Our closest large neighbor galaxy Andromeda also
shows evidence for past galactic astronomy. However, in both cases
these conclusions are inferred from "post-digestive" observations.

The destruction of dwarf galaxies is difficult to observe because dwarf
galaxies are inherently faint and their light becomes increasingly
diffuse as stars get pulled away by a larger galaxy. The only previously
known observation of the destruction of a dwarf galaxy in progress is
from the Advanced Camera for Surveys on the Hubble Space Telescope.

Taniguchi, Sasaki, Scoville and colleagues serendipitously discovered
the large elliptical galaxy (COSMOS J100003+020146) pulling apart the
dwarf galaxy (COSMOS J095959+020206) while observing an area of sky in
the constellation Sextans to study the properties of galaxies over large
scales in space and time. The pair of galaxies is about one billion
light years away and the distance between the two galaxies is about 330
thousand light years.

The thin band of stars extending from the dwarf galaxy both toward and
away from the large elliptical galaxy reveals that the gravity of the
elliptical is tidally tearing the dwarf apart. Stars that are closest to
the elliptical galaxy experience a stronger pull than stars in the
center of the dwarf galaxy, and stars on the opposite side experience a
weaker pull. As a result, the dwarf galaxy becomes stretched and looks
as if it's being pulled from two opposite directions even though there
is only one galaxy doing the pulling. This effect is comparable to how
two areas on the opposite sides of Earth experience high tide at the
same time even though there is only one Moon tugging on Earth's
oceans.

The tidally torn strip of stars in the newly observed pair of galaxies
is five times more extended and three times fainter in surface
brightness than the one observed with Hubble Space Telescope. Subaru
telescope's ability to gather large amounts of light and focus it into a
superbly sharp image was essential for this new discovery.

As astronomers find more examples of galactic cannibalism in action, our
knowledge of the history of galaxies should become increasingly
vivid. Although no human alive today will be able to witness the
ultimate of fate of the newly discovered pair, chances are the
elliptical galaxy will be able to complete the meal it's begun and fully
consume its neighbor.

Click each photograph to get its big image.

The giant elliptical galaxy in the left hand side is
COSMOS J100003+020146. The small galaxy that is linked to
this giant galaxy with the faint tidal tail is
COSMOS J095959+020206. Another tidal tail can also be seen
at the opposite (right hand) side of this small galaxy.

The same photograph but without scale.

The Andromeda galaxy (M 31) is shown at the left corner for reference
(Sandage & Bedke 1994, The Carnegie Atlas of Galaxies (Washington:
Carnegie Inst.)

The same photograph but without scale.

The first galaxy threshing system found by Dr. Duncan Forbes's group
(Forbes, D. et al. 2003, Science, 301, 1217)